One drawback of the simple two-stroke engine is a loss of a portion of the fresh unburned fuel charge from the cylinder during the scavenging process. Several concepts and technologies have been proposed or tried to circumvent the short-circuit loss of fresh charge. Air-head scavenging systems disclosed in U.S. Pat. Nos. 4,821,787, 6,112,708, 7,210,433, and 6,367,432 describe reed valve controlled air passages in air-head scavenged two-stroke engines. U.S. Pat. Nos. 7,363,888, 6,973,899, 7,025,021, 6,895,910, 6,289,856, 7,331,276, 6,564,760, and 6,497,204 describe piston controlled air head scavenging. However, all have imitations to achieving the best and cost effective method of making the engine with best the location of the ports, geometry of cavity and passages in the piston, geometry of the transfer passages in the cylinder. And also have limitations with respect to the crankshaft do not meet the configuration necessary to have the exhaust port in line with the crankshaft as packaged by, for example, Echo brand chainsaw. Secondly, the inlet of air requires dual ports to supply air to transfer passages on either side of the exhaust port. The U.S. Pat. Nos. 7,331,276, 7,363,888 and 6,564,760 describe cavity or window in the piston at two ends of the piston pin. The window or the cavity aligns with the transfer port and the cavity is significantly perpendicular to the axis of the piston pin. The piston and the cylinders disclosed in prior arts have cavity/window on the piston skirt and two separate air inlet ports in the cylinder one for each of the cavities. The invention disclosed by Mavinahally has one inlet passage in the cylinder aligning with one cavity in the piston and has internal air channel/passage in the piston either perpendicular to the piston pin or parallel to the pin as disclosed in this application. An air channel/passage inside of the piston interconnects the two cavities. In another embodiment disclosed here, the air passage connecting the two cavities/windows in the piston is above the piston pin and below the piston crown also known as piston top. The crown forms one of the walls for the passage. Therefore, the air passing from one cavity to the other cavity cools the piston crown. Therefore, it is advantageous to have at least a fraction of the air passing under the piston crown. Alternatively, hallow piston pin 200 can act as an air channel without needing or supplementing the separate air channel in the piston. The advantage with the air passage within the piston is that it offers the shortest air path between the two windows (cavities) in the piston and also offers shortest distance between the sets of transfer passages on either sides of the exhaust port. The air channel/passage offers shortest air path from the air inlet port from ambient into the transfer passage farthest from the air inlet port 98, when only one air inlet port is used in comparison to the prior arts. Secondly, in the embodiment described here, the air path from ambient into the crankcase chamber has unequal lengths of air paths from the air inlet port to the crankcase ports of transfer passages. The path through the transfer passage closest to the air inlet port is shorter than the air path through the transfer passage farthest from the air inlet port. However, the lengths of the flow paths for the air and the air-fuel mixture during the scavenging process is about equal for the both the sets of transfer passages. Having only one air inlet has the advantage of widening the air-fuel inlet port and also off-setting the air-fuel inlet port as necessary with respect to the plane passing through the axis of the cylinder and perpendicular to the piston pin, without limitations as in prior arts. Another advantage of having the passage parallel to the piston pin 200 is that the die casting of the piston is easier as the core for the piston pin bore, window 100a and 100b, and air passage 96 can be pulled in the same direction. As result a simpler and low cost die can be made. Secondly die casting of the internal air passage parallel to the piston pin can be machined on the same machine set up, and when the passage is a tubular passage, same tool can be used for machining the pin bore and the air passage. Advantage of having air inlet passage 313 and air-fuel inlet passage 312 perpendicular to the pin 200 is that the cooling air flowing across the cylinder from the fly-wheel or from front of the engine is obstructed as in the case where the inlet passage is in the front. Secondly, packaging of the fuel system and exhaust muffler in a hand-held engine as in a trimmer is easier and compact. Overall length of the engine including the muffler and carburetor and air filter is shorter when measured in the direction of the crankshaft when the inlet passages are perpendicular to the piston pin 200.
U.S. Pat. No. 5,379,732 teaches the art to vary the length of the transfer passage in a telescopic manner. U.S. Pat. Nos. 7,093,570 and 7,210,433 describe the transfer passages on the side walls (not the radial wall) of the crank case and on the flange between the crankcase and the upper cylinder block. U.S. Pat. No. 6,491,006 describes transfer passage which requires lid on the outside of the cylinder that can increase the cost of manufacturing the engine and secondly increases the risk of leaking fuel into ambient if the lid is not tightly fitted on to the engine or if the gasket or fastener holding the lid is broken.
U.S. Pat. No. 6,848,399 describes a scavenging conduit, also known as transfer passage, having an insert (100 (also known as cover) retained main crankshaft bearing. The insert may include a curve along the length of the longitudinal axis (of the cylinder, obvious from the description and the Figures) and the body 168 is circular in shape (col 4, line 4-8). It is also clear and obvious from FIGS. 1 and 2 that the insert stops above the bearing and is supported by the bearing. If it is extended below the cylinder, as shown in FIG. 1, the insert would be hit by the rotating (crank web) crankshaft 155 (col 2, line 33). Secondly, it becomes impossible to extend the length of the scavenging conduit (transfer passage) past the top of the main crankshaft bearing along the length of the longitudinal axis. If it is extended below the cylinder, as shown in FIG. 1 of prior art, the insert would be hit by the rotating (crank web) crankshaft 155 (col 2, line 33). The reason is that the main bearing and the crankshaft will interfere with the insert and damage the insert during operation of the engine. Thirdly the main bearing is a physical limitation for the extension of the insert described in the prior art. It is also obvious from the FIG. 1 and FIG. 2 that the insert outer surface 175 has a curvature in one dimension only, that is on a plane perpendicular to the axis of the cylinder (as described that the insert has a curve along the axis of the cylinder). It is also clear and obvious from FIGS. 1 and 2 that the insert stops above the bearing. The disadvantages of this prior art is that the length of the insert is restricted and limited by the main crankshaft bearing. Also, the length of the conduit is shorter and limited by the crank web (crankshaft 155) and the bearings. Extending the length into the crankcase as disclosed in the referenced prior art is impossible as it is obvious that the crankshaft bearing is in the way. In some applications, the length of the transfer passage is important and as disclosed in prior art by Nagesh Mavinahally and others, the transfer passage can extend into the crankcase chamber internally with the combination of cylinder and crankcase being cast such a way as to provide an enclosed conduit (transfer passage), and it does not have the insert. Therefore the prior art offers only a limited advantage and does not provide a solution to having a longer transfer passage (conduit). The prior art does not disclose a method to provide a spiraling channel. Also, the embodiment disclosed here offers a single or multiple but continuous insert that extends from the transfer port in the cylinder all the way, including a portion of, into the crankcase chamber below the cylindrical section of the cylinder.
It is desirable to have a spiraling transfer passage (channel/conduit) for more than two reasons; a) to increase the length of the passage within the length of the cylinder above the crankcase and above the centerline of the crankshaft, and b) to achieve the toroidal vertices in the flow of the charge into the combustion chamber during the scavenging process. Toroidal vertice is achieved by virtue of spiraling geometry of the transfer passage disclosed in the embodiment. Also, the spiraling passage directs the flow of the charge farther away from the exhaust port, and therefore reduces exhaust emissions significantly. Therefore, the embodiments disclosed in the invention, overcome the limitations of the prior arts and provide a cost effective design to overcome short comings of the prior arts. The embodiments disclose spiraling transfer passage and an insert to make a cylinder for stratified engines cost effectively and to induce toroidal flow into the combustion during the scavenging process.